The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a substrate carrier head and retaining ring of a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the surface of the substrate on which deposition occurs, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier (polishing head). The exposed surface (the lower surface as when the substrate is held in the polishing head) of the substrate is placed against a rotating polishing pad. The polishing pad may be a xe2x80x9cstandardxe2x80x9d pad in which the polishing pad surface is a durable roughened surface, or may be a fixed abrasive pad in which abrasive particles are held in a containment media. The polishing head provides a controllable load, i.e., force, on the substrate which pushes the substrate against the polishing pad. A polishing slurry is supplied to the polishing pad. The slurry includes at least one chemically-reactive agent, and, if a standard pad is used, includes abrasive particles is supplied to the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the polishing pad.
A reoccurring problem in CMP is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency for the edge of the substrate to be polished at a different rate than the center of the substrate. The edge effect typically results in over-polishing (the removal of too much material from the substrate) of the perimeter portion, e.g., the outermost five to ten millimeters, of the substrate. The over-polishing of the substrate perimeter reduces the overall flatness of the substrate, makes the edge of the substrate unsuitable for use in integrated circuits, and decreases the yield.
In view of the foregoing, there is a need for a chemical mechanical polishing apparatus which provides the desired surface flatness and finish while minimizing the edge effect.
According to one aspect, the invention provides a retaining ring for use with a substrate polishing apparatus. The substrate has upper and lower faces and a perimeter. The polishing apparatus has a movable polishing pad with an upper polishing surface for contacting and polishing the lower face of the substrate. The retaining ring has a retaining face for engaging and retaining the substrate against lateral movement, and has a bottom face for contacting the polishing surface of the polishing pad. The bottom face of the retaining ring descends from an inner portion adjacent the retaining face to a lowermost portion radially outboard of the retaining face.
Implementations of the invention may include one or more of the following. The lowermost portion may be approximately 5-15 millimeters outboard of the retaining face. The lowermost portion may be approximately 10 millimeters outboard of the retaining face. The lowermost portion may be approximately 0.5 to 2.0 millimeters below an intersection of the bottom face and the retaining face. The lowermost portion may be approximately 1 millimeter below an intersection of the bottom face and the retaining face. The bottom face may ascend from the lowermost portion to an outer portion radially outboard of the lowermost portion. An intersection of the retaining face and the inner portion of the bottom face may be at a substantially even level with the lower face of the substrate when the retaining face engages the substrate.
According to another aspect, the invention is directed to a retaining ring for use in conjunction with an apparatus for polishing a substrate. The substrate has upper and lower faces and a lateral perimeter. The apparatus has a polishing pad with an upper polishing surface for contacting and polishing the lower face of the substrate. The retaining ring has an inner face for surrounding and engaging the substrate perimeter. The retaining ring has a bottom face extending outward from the inner face for contacting the polishing surface of the polishing pad. The bottom face of the retaining ring has an annular downward facing convex region.
Implementations of the invention may include one or more of the following. The bottom face of the retaining ring may have an annular downward facing concave region inboard of the annular downward facing convex region. The bottom face of the retaining ring may have a second annular downward facing concave region outboard of the annular downward facing convex region. The bottom face of the retaining ring may have a annular downward facing flat horizontal region inboard of the annular downward facing convex region. The retaining face may be substantially vertical and the ring may further comprise a vertical outboard face. The bottom face of the ring may connect the retaining face and the outboard face, and have a first annular intersection with the retaining face and a second annular intersection with the outboard face. The first annular intersection may be located at a lower height than the second annular intersection.
According to another aspect, the invention has a retaining ring having an inward facing retaining face for engaging and retaining a substrate against lateral movement, and a bottom face for contacting the polishing surface of a polishing pad. The bottom face has a downward projecting lip, which projects below the lower face of the substrate.
According to another aspect, the invention has a polishing head for holding a substrate in engagement with a movable polishing pad. The head has a housing and a substrate backing member for engaging an upper surface of the substrate. The substrate backing member is vertically movable relative to the housing for maintaining a lower surface of the substrate in engagement with an upper surface of the polishing pad. A retaining ring is vertically movable relative to the substrate backing member and has an inward facing retaining face for engaging and retaining the substrate against lateral movement. The retaining ring has a bottom face for contacting the upper surface of the polishing pad. The bottom face descends from an inner portion adjacent the retaining face to a lowermost portion radially outboard of the retaining face. The bottom face of the retaining ring may ascend from the lowermost portion to an outer portion, radially outboard of the lowermost portion.